Minimally invasive surgical techniques have gained significant popularity due to their ability to accomplish desirable outcomes with reduced patient pain and accelerated recovery and return of the patient to normal activities. Arthroscopic surgery, wherein the intra-articular space is filled with fluid, allows orthopedic surgeons to efficiently perform procedures using special purpose instruments designed specifically for arthroscopy. Among these special purpose tools are various manual graspers and biters, powered shaver blades and burs, and electrosurgical devices. During the last several years, specialized arthroscopic electrosurgical electrodes referred to in the art as “ablators” have been developed. Examples of such instruments include “ArthroWands” manufactured by Arthrocare (Austin, Tex.), VAPR electrodes manufactured by Depuy Mitek (Raynham, Mass.) and electrodes by Smith and Nephew, Inc. (Andover, Mass.). These ablator electrodes differ from conventional arthroscopic electrosurgical electrodes in that they are designed for the bulk removal of tissue by vaporization rather than the cutting of tissue or coagulation of bleeding vessels. While standard electrodes are capable of ablation, their geometries are generally not efficient for accomplishing this task. The tissue removal rates of ablator electrodes are lower than those of arthroscopic shaver blades, however, electrosurgical ablators are used because they achieve hemostasis (stop bleeding) during use and are able to efficiently remove tissue from bony surfaces. Ablator electrodes are used in an environment filled with electrically conductive fluid.
During ablation, current flows from the ablator into the conductive fluid and locally heats the fluid to its boiling point. The relative heating of the conductive fluid is proportional to the density of electrical current flowing from the electrode into the fluid. Regions of high current density will experience higher rates of heating as compared to regions of low current density. In general, regions of high current density occur at the corners and edges of the electrode. Steam bubbles form first at the edges of an ablator but eventually cover virtually the electrode's entire exposed surface. When a steam bubble reaches a critical size, arcing occurs within the bubble and enclosed portion of tissue. A train of sparks occurs within the bubble with the train ending when the bubble grows too large or the tissue enclosed in the bubble is evaporated and conditions within the bubble become unfavorable for sparking.
During ablation, water within the target tissue is vaporized. Because volumes of tissue are vaporized rather than discretely cut out and removed from the surgical site, the power requirements of ablator electrodes are generally higher than those of other arthroscopic electrosurgical electrodes. The efficiency of the electrode design and the characteristics of the Radio Frequency (RF) power supplied to the electrode also affect the amount of power required for ablation. Electrodes with inefficient designs and/or powered by RF energy sources with poorly suited characteristics will require higher power levels than those with efficient designs and appropriate generators. Because of these factors, the ablation power levels of devices produced by different manufacturers vary widely with some using power levels significantly higher than those commonly used by arthroscopists. Ablator electrode systems from some manufacturers may use up to 400 Watts, significantly higher than the 30 to 70 Watt range generally used by other arthroscopic electrosurgical electrodes.
During arthroscopic electrosurgery, all of the RF energy supplied to the electrode is converted into heat, thereby raising the temperature of the fluid within the joint and the temperature of adjacent tissue. Prior to the introduction of ablator electrodes, the temperature of the fluid within the joint was not of concern to the surgeon. However, due to the higher power levels at which they generally operate and the longer periods of time that they are energized, fluid temperature is a major concern during the use of ablator electrodes. Standard arthroscopic electrosurgical electrodes are usually energized for only brief periods, generally measured in seconds, while specific tissue is resected or modified, or a bleeder coagulated. In contrast, ablator electrodes are energized for longer periods of time, often measured in minutes, while volumes of tissue are vaporized.
The temperature of the fluid within the joint is critical since nerve damage and cell death can occurs at tissue temperatures as low as 45-50° C., a temperature easily reached with high-powered ablators if fluid flow through the surgical site is insufficient. Patient injury may result. Such injuries have been documented.
The likelihood of thermal injury is strongly affected by the amount of power supplied to the ablator. This, in turn, is determined by the efficiency of the ablator and the speed with which the surgeon desires to remove tissue. A highly efficient ablator will allow the surgeon to remove tissue at desirably high rates while requiring low levels of power input. Under these conditions, the likelihood of thermal injuries is reduced significantly.
Ablator electrodes are produced in a variety of sizes and configurations to suit a variety of procedures. For example, ablators designed for use in ankle, wrist or elbow arthroscopy are generally smaller than those used in the knee or shoulder. Each size embodiment is then produced in a variety of configurations to facilitate access to various structures within the joint being treated. These configurations differ in terms of the working length of the electrode (i.e., the maximum distance that an electrode can be inserted into a joint), the size and shape of the ablating surfaces and the angle between the ablating face and the axis of the electrode shaft. Electrodes are typically designated by the angle between a normal to the ablating surface and the axis of the electrode shaft, and by the size of their ablating surface and any associated insulator.
Primary considerations of surgeons when choosing a particular configuration of ablator for a specific procedure include its convenience of use (i.e., the ease with which the instrument is able to access certain structures) and the speed with which the ablator will be able to complete the required tasks. When choosing between two configurations capable of accomplishing a particular task, surgeons will generally choose the ablator with the larger ablating surface so as to remove tissue more quickly. This is particularly true for procedures during which large volumes of tissue must be removed. One such procedure is acromioplasty, the reshaping of the acromion. The underside of the acromion is covered with highly vascular tissue that may bleed profusely when removed by a conventional powered cutting instrument, such as an arthroscopic shaver blade. Ablator electrodes are used extensively during this procedure since they are able to remove tissue without the bleeding which obscures the surgeon's view of the site. Ablation in the area under the acromion is most efficiently accomplished using an electrode on which a line normal to the ablating surface is approximately perpendicular to the axis of the ablator shaft. Such an electrode is referred to in the field as a “90 Degree Ablator” or a “side effect” ablator. Examples of such electrodes include the “3.2 mm 90 Degree Three-Rib UltrAblator” by Linvatec Corporation (Largo, Fla.), the “90 Degree Ablator” and “90 Degree High Profile Ablator” by Smith and Nephew (Andover, Mass.), the “Side Effect VAPR Electrode” by Depuy Mitek (Raynham, Mass.), and the “3.5 mm 90 Degree Arthrowand”, “3.6 mm 90 Degree Lo Pro Arthrowand”, and “4.5 mm 90 Deg. Eliminator Arthrowand” by Arthrocare Corporation, and “3 mm OPES Ablator” and “4 mm OPES Ablator” and others by Arthrex (Naples, Fla.).
Recently ablator electrodes have been configured with mechanism and means for removing bubbles and debris from the surgical site. During electrosurgery in a conductive fluid environment, tissue is vaporized producing steam bubbles that may obscure the view of the surgeon or displace saline from the area of the intra-articular space that the surgeon wishes to affect. In the case of ablation (i.e., bulk vaporization of tissue), the number and volume of bubbles produced is even greater than when using other electrodes since fluid is continually boiling at the active electrode during use. Ideally, flow through the joint carries these bubbles away; however, in certain procedures, this flow is insufficient to remove all of the bubbles. Accordingly, the ablator is configured with an aspiration means that removes some bubbles as they are formed by the ablation process, and others after they have collected in pockets within the joint. The ablator aspiration means is typically connected to an external vacuum source that provides suction for bubble evacuation. An illustrative example of an aspirating ablator is described by Carmel, et al. in U.S. Pat. No. 7,837,683 issued Nov. 23, 2010, the contents of which are incorporated by reference herein. While Carmel suggests positioning an aspiration port in the center of the active electrode, other aspirating schemes are contemplated. See, for example, the flexible aspirating ablators described in a co-pending application to Van Wyk, U.S. Ser. No. 13/091,584 filed Apr. 21, 2011, the contents of which are incorporated by reference herein.
The construction of ablators may be generally separated into two categories: (a) those with simple construction in which the RF energy is conducted to the active electrode by the distally extending structural member, and (b) those with complex construction in which the RF energy is conducted to the active electrode by wires within a tubular distally extending structural member. Examples of simple construction ablators include the monopolar devices marketed by Linvatec, Arthrex and Smith and Nephew. Examples of complex construction ablators include those marketed by Arthrocare, DePuy Mitek, Stryker Corporation (San Jose, Calif.) and the bipolar ablators marketed by Smith and Nephew. All bipolar devices are necessarily categorized as a “complex construction” necessitated by the presence of both active and return electrodes at the vicinity of the electrode distal tip.
Arthroscopic ablators have a distal end construction in which an active electrode is surrounded by a ceramic insulator that covers the active electrode, with the exception of the exposed ablating surface that generally protrudes beyond the insulator a short distance. The axial positioning of the insulator generally is maintained by a flange on the active electrode, the flange typically having a distally facing surface against which the proximal end of the insulator is positioned. The insulator is generally held in place by an adhesive (typically, an epoxy) and/or by a dielectric coating that covers the elongated distal element of the ablator and overlaps the proximal end of the insulator. The dielectric coating is frequently applied as a powder that is then fused to the device by curing at an elevated temperature.
Heat from the ablating arcs heats the active electrode. Indeed, the arcs vaporize portions of the active electrode on which the arcing occurs such that the ablating surface and its features are eroded during use. Heat from the arcs flows into the active electrode raising the temperature of the active electrode and the adjacent insulator. In the case of ablators of simple construction, wherein the insulator is retained on the assembly by an adhesive or polymeric coating, the local elevation of electrode and insulator temperatures may result in the melting or degradation of the adhesive or coating. This is particularly true in the region in which the proximal face of the insulator contacts the flange of the active electrode, as the protruding discontinuity of the active electrode surface at this location tends to result in a concentration of the electric field. High temperatures at this location, combined with the intensified electric field, may cause a premature breakdown of the insulating polymeric coating so that arcing occurs between the underlying electrode at this location and the conductive fluid surrounding the ablator. After initial breakdown of the insulating coating, arcing at the location destroys adjacent coating and the opening in the coating grows. Arcing at this location causes additional heating of the active electrode and insulator frequently leading to catastrophic failure of the coating. This catastrophic failure frequently destroys the bond between the active electrode and the insulator, thereby allowing the insulator to fall off of the assembly into the patient's joint. The high temperature resulting from coating failure may also cause thermal gradients within the insulator that may, in turn, cause it to break apart. As accident reports documented by the database maintained by the Food and Drug Administration (FDA) demonstrate, insulator pieces frequently fall into the patient's joint. When the insulator or parts thereof fall into a patient's joint space, the surgeon must retrieve the foreign bodies from the site. This can be easily accomplished if the foreign bodies are ejected into a site where they are visible. However, in many cases, the pieces fall into locations that are hidden from view and the surgeon must do an extensive search, a process that frequently involves bringing an imaging system into the operating room or, in some cases, converting the minimally invasive procedure to a full open surgical procedure. The task of retrieving pieces that fall into the patient body is further complicated by the fact that such pieces cannot be easily detected by various X-ray or fluoroscopy imaging systems. This can cause lengthy delays in the surgery, and in some cases, can result in the insulator or insulator fragments remaining in the body of the patient after the surgery. Obviously, neither of these unintended consequences is desirable.
To prevent failure of the polymeric coating and adhesive, and therefore to increase patient safety by increasing the reliability of electrosurgical devices, it is desirable to use coatings and adhesives with high service temperatures. However, the selected material must also be biocompatible. The dielectric coating that covers the elongate distal member and overlaps the proximal end of the insulator may be applied electrostatically as a powder and cured at elevated temperature or, alternatively, may be a tubular “heat shrink” material, i.e., an extruded polymeric tube that is positioned on the device and then shrunk in place using heat. While a cured powder coating is able to both cover the tube and provide a means for retaining the insulator in position, use of the heat shrink material requires the use of an additional adhesive to retain the insulator in position. If the adhesive fails due to excessive heating during use, the heat shrink tubing provides little retaining force on the insulator, and the insulator is thus easily ejected into the joint. Nevertheless, biocompatible heat-shrink materials are available with high dielectric strengths and service temperatures higher than those of biocompatible powder coatings thereby making the use of heat-shrink materials desirable on electrosurgical devices when possible.
The temperature of the active electrode/insulator assembly may be minimized through efficient dispersal of heat from the active electrode. With non-aspirating electrodes, the heat may be conducted proximally into the elongate member by maximizing the device cross-section proximal to the insulator so as to allow effective heat conduction. In the case of aspirating ablators, the flow through the device may be maximized to provide effective convective cooling of the electrode. The flow is limited, however, by the size of the aspiration port, and cooling of the assembly may be limited by the presence of any dielectric coating on the inside of the aspiration lumen.
A novel means to prevent ejection of an insulator from an electrosurgical ablator into a patient is described in DeCesare et al. in U.S. Pat. No. 7,150,746 issued Dec. 19, 2006. Therein, a flange is provided on the distal portion of the active electrode to prevent the insulator from slipping distally off of the active electrode in the event of coating failure. While this is effective, it requires a unique construction not applicable to all types of ablators, and further requires increasing the size of the device distal end beyond what may be desirable in many applications.
Indeed, it is desirable to make the distal end of an ablator as small as is practically possible so as to minimize the requisite diameter of the introductory canola and thereby minimize trauma to the joint space. Also, ablating tissue in tight spaces like the wrist and elbow requires the use of a small ablator, the smaller sizes affording the surgeon with greater maneuverability in the joint space. Accordingly, the design options for an arthroscopy ablator are limited by thermal concerns and the inability to use mechanical fastening means to affix the insulator to the active electrode except in extremely limited circumstances.
However, the afore-mentioned problems are not unique to arthroscopy ablators. Other medical applications require the construction of electrosurgical devices that are very small in size, yet reliable and safe for the patient. For instance, in the field of urology, in the context of treating benign prostatic hyperplasia (BPH), a condition commonly referred to as “enlarged prostate”, the ablators must be sufficiently small to pass through the lumen of a resectoscope inserted into the urethra of a patient. One such device is by Carmel et al. in WO 2008/039746 published Apr. 3, 2008, the contents of which are incorporated by reference herein. These resectoscopes generally have lumens measuring less than 0.3 inches (7.5 mm) and accommodate an optic used to view the surgical site as well as the ablator used to treat tissue. As with arthroscopy ablators, the portion of the active electrode element forming the ablating surface is exposed and the rest of the element is covered by an insulator, preferably fabricated of a non-conductive ceramic material. Because the electrode assembly of an ablator used in a resectoscope is mounted to the distal end of two wires of small cross-section, there is little conductive removal of heat from the assembly, and aspiration flow is not present.
Accordingly, the electrode assembly of an electrosurgical device for tissue vaporization and coagulation used with a resectoscope is frequently heated to temperatures higher than those of arthroscopy ablators, beyond the temperature at which polymeric adhesives provide a reliable bond between the active electrode and insulator. Because the assemblies are very small, it is extremely difficult to mechanically affix the insulator to the active electrode, and doing so unacceptably limits design choices. The problem is compounded if the device is bipolar, with the return electrode also mounted to the insulator, or if the device employs a “floating electrode” as described by Carmel in the above-referenced pending application or in the context of U.S. Pat. Nos. 7,563,261 and 7,566,333, issued Jul. 21, 2009 and Jul. 28, 2009, respectively, the contents of which are incorporated by reference herein.
Accordingly, there is a significant need in the art to improve the affixation of insulator to electrode in the context of electrosurgical devices, particularly those adapted for the modification, sculpting, resection, removal, or vaporization of tissue, configured for coagulation, cauterization or hemostasis purposes, or utilized for thermal treatment of tumors as well as normal tissues. The process should allow the use of heat-shrink materials for insulating of the elongate distal element and proximal portion of the electrode element. At the same time, there is a significant need in the art for the miniaturization of electrosurgical devices without sacrificing the safety and reliability of the device. This is especially important in the context of ablation devices, since the tendency of many electrosurgical vendors is to employ higher and higher electrical power levels, currently as much as 400 Watt, in order to achieve the desired clinical results.